In 1938, in Egypt a team of archaeologists are digging into a tomb to
look for a valuable artifact. A young boy falls through a hole that
leads his father and the others to what they came for - the tablet of
Ahkmenrah. Several local men see the tablet and say that if the
archaeologists remove it, then "the end will come".

Larry meets a new wax figure Neanderthal
named Laa who was modeled to look like Larry as a joke. Ahkmenrah had
earlier alerted Larry to the fact that the tablet is starting to
corrode, causing all the exhibits to act abnormally. The corrosion
causes chaos during the event, with the attendees having to flee from
dangers caused by the affected exhibits. When Larry returns home, he
catches his son Nick throwing a party for which he is serving as the DJ.
Nick tells Larry that he does not intend to apply to college, wanting
to take time to pursue being a DJ and to figure out what he wants to do
with his life.

Larry researches the tablet and learns that Cecil Fredericks, the
former museum security guard that tried to steal the tablet for himself,
was the boy present at the discovery of the tablet. Larry finds Cecil
in a retirement home along with his cohorts Gus and Reginald. Larry
explains to Cecil what is happening and he recalls "the end will come"
prophecy. He suggests that they consult Ahkmenrah's parents, who are in
the British Museum.
Larry later goes to McPhee, who is in the process of being fired, and
convinces him to call the British Museum and allow Larry to travel there
with Ahkmenrah and the tablet.

Larry and Nick travel to London to get to the British Museum. They
meet the security guard of that museum, Tilly, who lets them through the
gate. When Larry gets into the museum, he sees that Laa, Teddy,
Sacagawea, Attila, Dexter, Jed, and Octavius have all stowed away to
join on the adventure. The Tablet brings the British exhibits to life
and the gang encounters a Triceratops skeleton which chases them. They are saved by Sir Lancelot.
Unfortunately, the gang has lost Jed and Octavius; they discover that
they have fallen into the ventilation chambers. Larry sends Dexter into
the ventilation with Nick's cell phone attached to his waist to find Jed
and Octavius. Jed and Octavius land on a Pompeii exhibit just before it erupts. The gang fights off a nine-headed Xiangliu statue, which Larry defeats using a defibrillator. Meanwhile, Dexter urinates on the lava to save Jed and Octavius.

The gang finds the tombs of Ahkmenrah's parents, Merenkahre and
Shepseheret. Merenkahre is at first hesitant to reveal the truth behind
the Tablet, but he eventually admits that it was a gift made for his son
when he was a baby to keep the family together always. Endowed with the
power of Khonsu, the tablet needs moonlight to retain its magic.

Lancelot steals the tablet, thinking it to be the Holy Grail
and then leaves to find Guinevere. The tablet continues to corrode,
causing Teddy's hands to return to wax and Ahkmenrah to age. The gang
splits up to find Lancelot and regain the tablet, but Larry is caught by
Tilly, who then locks Larry and Laa in the security room. There, Larry
starts to think about his relationship with Nick. Laa eventually smashes
the glass of the door with his head to free Larry and himself. Larry
and the gang continue to search for Lancelot again, but are cornered by
the Trafalgar Squarelion statues. Larry distracts them with a flashlight like in the cat video Jed and Octavius had previously watched on YouTube.

Lancelot has made his way to a local theater showing a Camelot production starring Hugh Jackman as King Arthur and Alice Eve as Guinevere.
Sir Lancelot runs onstage with the tablet and tries to convince
"Guinevere" to join him. The actors eventually convince Lancelot that
they are actors, upsetting Lancelot and causing him to take a lighted
torch from the stage and run to the roof. Larry and the gang chase
Lancelot and attempt to get him to adjust the tablet appropriately in
order for the moonlight to restore it. Larry convinces Lancelot to give
him back the tablet to save his friends once the other exhibits all
return to their lifeless state due to the corrosion of the tablet. Larry
then adjusts the tablet and it is restored by the moon, reanimating
Teddy, Dexter, and the rest.

Larry returns the tablet to Merenkahre after the New York exhibits
explain that they believe the tablet and the pharaohs should stay
together. On the way out, Larry catches Tilly and Laa in a romantic
moment. As they leave, Larry tells Tilly that, starting the next day,
she will have the greatest job in the world. Back in New York, the
exhibits accept their fate and say goodbye to Larry.

Three years later, a British Museum treasures exhibition event is
happening at the Museum of Natural History. Tilly shows up to McPhee's
office with a box. McPhee explains that Larry left the museum and went
on to become a teacher, after taking the blame for what happened when
the exhibits went ballistic, granting McPhee his job back. Tilly opens
the box and takes out the tablet. She brings McPhee to show him all the
exhibits coming to life, including some new light displays. Outside,
Larry watches and smiles.